Conventional electric linear actuators can be classified into, for example, three types in accordance with the driving system. In the description given below, the three types of conventional electric linear actuators are referred to as a first conventional actuator 910, a second conventional actuator 920, and a third conventional actuator 930.
FIG. 22 illustrates an example of the first conventional actuator 910. The first conventional actuator 910 includes a casing 911, a fixed element 912, a movable element 916, and a movable element spring 917.
The casing 911 accommodates an output shaft 918 of an electric device, the fixed element 912, the movable element 916, and the movable element spring 917. The fixed element 912 is fixed to the casing 911. The fixed element 912 includes a coil 913, two permanent magnets 914, and four yokes 915. The coil 913, the permanent magnets 914, and the yokes 915 each have an annular shape. The movable element 916 is fixed to the output shaft 918. The movable element spring 917 is arranged between the casing 911 and the movable element 916. The movable element spring 917 applies force, acting in an axial direction of the output shaft 918, to the movable element 916.
The first conventional actuator 910 switches the direction of current supplied to the coil 913 to reciprocate the movable element 916 and the output shaft 918. However, in the first conventional actuator 910, unwanted vibration occurs due to the inertial force of the movable element 916. Patent Document 1 discloses an example of the first conventional actuator 910.
The second conventional actuator 920 includes a technique of reducing the unwanted vibration of the movable element 916. FIG. 23 illustrates an example of the second conventional actuator 920. The second conventional actuator 920 has a structure substantially the same as that of the first conventional actuator 910 except in that a weight 921, an intermediate spring 922, and a weight spring 923 are provided.
The weight 921 is arranged around a portion of the output shaft 918. The weight 921 moves relative to the output shaft 918 in the axial direction. The intermediate spring 922 is arranged between the movable element 916 and the weight 921. The intermediate spring 922 applies force, acting in the axial direction of the output shaft 918, to the movable element 916 and the weight 921. The weight spring 923 is arranged between the casing 911 and the weight 921. The weight spring 923 applies force, acting in the axial direction of the output shaft 918, to the weight 921.
The second conventional actuator 920 supplies the coil 913 with current, having a frequency close to the natural frequency in a secondary vibration mode. By supplying the current to the coil 913, the second conventional actuator 920 reciprocates the movable element 916 and the weight 921 in opposite phases. Thus, the unwanted vibration of the movable element 916 is reduced. However, the second conventional actuator 920 includes the weight 921 and thus has a larger size than the first conventional actuator 910. Patent Document 2 discloses an example of the second conventional actuator 920.
The third conventional actuator 930 includes a technique with which a smaller size and a higher efficiency can be achieved. FIG. 24 illustrates an example of the third conventional actuator 930. The third conventional actuator 930 includes an electromagnetic core block 931, two magnetic blocks 934, a block coupling member 937, a first coupling member 938, and a second coupling member 939.
The electromagnetic core block 931 is coupled to the block coupling member 937. The electromagnetic core block 931 includes a core 932 and a coil 933. Each magnetic block 934 is coupled to the block coupling member 937. Each magnetic block 934 reciprocates in the sideward direction as viewed in FIG. 24. The two magnetic blocks 934 are arranged in parallel in a direction (vertical direction as viewed in the drawing) orthogonal to the direction in which the magnetic blocks 934 reciprocate. FIG. 24 illustrates one of the two magnetic blocks 934. The other one of the magnetic blocks 934 (not shown) is arranged at a farther side of the illustrated magnetic block 934 in the vertical direction as viewed in the drawing. Each magnetic block 934 includes a permanent magnet 935 and a back yoke 936. The permanent magnet 935 and the electromagnetic core block 931 are opposed to each other with a gap in between.
The first coupling member 938 is arranged on one end side of each magnetic block 934 in the reciprocation direction of the magnetic block 934. The one magnetic block 934 and the other magnetic block 934 are coupled to each other by the first coupling member 938. The second coupling member 939 is arranged on the other end side of each magnetic block 934 in the reciprocation direction of the magnetic block 934. The second coupling member 939 couples the two magnetic blocks 934.
The third conventional actuator 930 supplies current to the coil 933 to reciprocate the two magnetic blocks 934 in opposite phases. This reduces unwanted vibration of the magnetic block 934. In the third conventional actuator 930, the two magnetic blocks 934 are arranged in parallel. Thus, the length in the reciprocation direction can be shortened compared with the second conventional actuator 920. Patent Document 3 illustrates an example of the third conventional actuator 930.